Ultrasonic Cavitation & Cavitating Flows

Cavitation is a common phenomenon which appears in e.g.pumps, turbines, fluid valves and nozzles. In the short term this leads to a drop in discharge, flow rate limitation and in the long term to abrasive erosion on pump blades and surfaces that are in closed contact to the cavitating fluid.
GMBU Halle is developing customised measurement concepts to record and to quantify cavitation intensity in power ultrasonic systems, fluidic devices and fluid machines non-invasively and ad hoc.

DETERMINATION OF CAVITATION UND CAVITATION INTENSITY BASED ON AE

The AE monitoring concept of GMBU is based on the non-invasive detection of cavitation noise using AE piezosensors installed at the bodies of devices and machines.
The concept was first tested on cavitation generated with power ultrasonic processors, using the sonotrode as a waveguide for the high-frequency AE-signals (DOWNLOAD Cavitation Noise Sensor). The averaged cavitation noise appears as a characteristic curve in the log-plot spectrum (s. Figure left). This signal patterns could be found analogously for hydrodynamically induced cavitation noise. Thus the recorded signals follow an universal scaling law, which is a theoretical founding of that concept.

TYPES OF CAVITATION STRUCTURES IN NOZZLES AND ORIFICE

Cavitation manifests itself in machines in a variety of fluid structures, e.g. layered cavitation, shedding of cavitation clouds and even supercavitation. Investigations on a convergent-divergent nozzle (Fig. below) show that photographically detectable cavitating structures can be depicted using recorded by AE signatures simultaneously. The temporal resolution of both methods ar in the same range. This opens the road to a noninvasive monitoring method for cavitation in fluidmachinery processes.

NON-INVASIVE DETECTION OF CAVITATION IN PUMPS UND TURBINES

The onset of cavitation in rotating fluid machines leads to the formation of cavitation bubble fields at the trailing edges of the impellers. These are dragged along with the rotating impeller or detach from it. Bubble fields that implode in the immediate vicinity of the surface lead to successive material damage and erosion (cavitation erosion). The cavitating bubbles are a source of noise.
The diverse acoustic effects of cavitation can be recorded with the aid of AE-Sensors or structure-borne sound sensors. Structure-borne sound sensors have a high frequency bandwidth and signal dynamics by its own. The signals are amplified and can be analysed in both time and frequency space. The higher the spectral bandwidth of the recorded signals, the better the signal resolution of the cavitation events (e.g. shockwaves of imploding single bubbles, bubble clouds and jet impact) and thus exploring what is happening at the individual rotating blade (see DOWNLOAD Cavitation in Turbines and Cavitation in Pumps)

FROM THE DETECTION OF CAVITATION INTENSITY TILL CAVITATION EROSION

Our vision is to establish the correlation between cavitation intensity and cavitation erosion based on AE measurements.
This opens the way for a non-invasive method for asses cavitation aggressivness linked to cavitation erosion. The aim here is also to make early statements on the damage potential of cavitating structures and thus improve the concepts for condition monitoring and predictive maintenance in fluid machinery processses.

REFERENCES

S. Gai, L. Ledig, U. Bauerschäfer. Möglichkeiten der Erfassung der Kavitation mit Hilfe der Akustischen Detektion, Vortrag, 48. Sitzung des Arbeitskreises Strömungsmaschinen, 08.04.2016, Universität Otto von Guericke in Magdeburg.

S. Gai, L. Ledig, U. Bauerschäfer, GMBU e.V, Halle, Germany, Sensing Cavitation in Hydraulic Machines and Devices by Acoustic Emissions, Presentation,Workshop on Cavitation Expoitaton, University of Ljubliana 27.-28 September 2018, Ljubliana, Slovenia

U. Bauerschäfer, S. Gai, L. Ledig, GMBU e. V.
Fortschritte bei der Erfassung von Kavitation in Pumpen und Turbomaschinen, Vortrag, 13. Tagung Technische Diagnostik 2018,HS Merseburg.

U. Bauerschäfer , S. Gai, J. Hufen, L. Ledig, Piezo-Transducer Grenzfläche als Sensor für Kavitationserosion, Vortrag, Workshop Komplexe Aspekte der Kavitation, Kloster Drübeck 13. November 2019

U. Bauerschäfer, S.Gai, L.Ledig, Analysis of cavitation noise at an ultrasonic horn tip using an integrated sensor, Posterpresentation, Workshop Cavitations meets Data Science .- Advances, apllications and challenges at the interfaces of nonlinear physics, data processing and machine learning, Int. Symposium Göttingen, 10./11.06.2022

U. Bauerschäfer, L. Ledig, K. S. Schulz, S. L. Gai, Untersuchung der Jet-Inversion von Laser generierten Kavitationsblasen an gefangenen freien Gas-Oberflächen unter Wasser,
Workshop Kavitation 01. bis 03.011 2023, Kloster Drübeck

U. Bauerschäfer, L. Ledig, K. Krueger, S. L. Gai, Cavitation Erosion Reduction through self-regenerating trapped immersed free Gas surfaces under water, 12th International Cavitation Symposium CAV2024 at the M??Ch Conference Centre in Chania, Greece.